The concept of scavenging or harvesting energy from the environment has long been utilized in the form of wind, water, and solar energy. More recently, by means of suitable transducers, other forms of energy can also be gathered and transformed into electrical energy.
Various materials are used in energy harvesters. For example, transducers utilizing piezoelectric materials use the piezoelectric effect to affect the change between electrical energy and flexural or vibration changes, see, e.g., U.S. Pat. No. 6,771,007. Also available are pyroelectric transducers, which use temperature changes to effect the generation of electrical power, see, e.g., U.S. Pat. No. 6,528,898.
The use of piezoelectric transducers to generate electrical power from movement has been a goal in the defense industry where energy can be generated simply from walking, via a heel-strike transducer, to power man-portable electronic devices, see, e.g., U.S. Pat. No. 6,768,246. Such self-powered devices have also been proposed as motion detectors to send a signal when significant vibration occurs, see, e.g., U.S. Pat. No. 5,801,475. In the biomedical field, M. J. Ramsay et al. proposed using piezoelectric transducers to harvest energy from biological functions, in this case, fluctuations in blood pressure, to generate small amounts of electrical power, see, “Piezoelectric Energy Harvesting for Bio MEMS Applications”, Smart Structures and Materials 2001: Industrial and Commercial Applications of Smart Structures Technologies, Anna-Maria McGowan (Ed.), Proceedings, SPIE Vol. 4332 (2001), p. 429.
In-tire electronics is another field where there has been much interest in energy scavenging and generation. Currently, tire monitoring devices are mounted within the wheel to monitor, for example, tire pressure, wheel imbalance, or tire wear. Powering these devices generally requires the use of batteries. The downside is that batteries have a restricted lifespan, add to the weight of the device, require additional wheel balancing, and do not operate well at the temperature extremes (−40° C. and 125° C.) encountered in this application.
Efforts have been made to investigate alternative methods of providing electrical power to these tire monitoring devices. One method, which is sometimes referred to as the passive approach, uses devices such as SAW filters. The filters are positioned in or on the wheel so that they can be activated by an external electromagnetic field and can then respond with information on the status of the wheel/tire, see, e.g., U.S. Pat. No. 5,585,571. Another method involves scavenging energy from the tire or wheel, in the form of vibrations or flexing of the tire wall. For example, U.S. Pat. No. 6,407,484 makes use of the motion of the tire; U.S. Patent Application Publication No. 20040100100 discloses a coil and magnet device attached to the wheel and the tire inner wall; and U.S. Pat. No. 6,847,126 harvests static electricity built up during the motion of the tire.
In addition, various designs and configurations for scavenging energy from the motion of the wheel or tire have been proposed, see, e.g., U.S. Pat. Nos. 6,725,713, 6,807,523, and U.S. Patent Application Publication No. 20040211250. These consist primarily of piezoelectric structures embedded within the tire, which are flexed when the tire rotates. U.S. Pat. No. 6,175,302 discloses mounting such a piezoelectric structure separately in a tire valve. As is known in the art, the electrical energy thus produced can be stored by a variety of means, see, e.g., U.S. Pat. No. 5,703,474.
As one skilled in the art will appreciate, there is a continuing need for better, more cost-effective power scavengers for use in tire monitoring devices. Accordingly, there is a need in the art to develop new, efficient power scavengers for use in these devices.